#include<iostream>
#include<string>
#include<algorithm>
using namespace std;

char a[100000]={0};


int main(){
    int n=0;
    string s;
    cin>>n>>s;
    int j=0;
    for(int i=0;i<s.size();){
        a[j++]=s[i];
        char b=s[i];
        while(s.find(b)!=string::npos){
            s.erase(s.find(b),1);
        }
        i=0;
    }
    sort(a,a+j);
    for(int i=0;i<j;i++){
        cout<<a[i];
    }

    return 0;
}
